Technical Abstract:
Extensive cutting management research has documented the effects of date and frequency of harvest on alfalfa (Medicago sativa L.) forage yield and quality. Information is lacking, however, on the change in quality relative to yield that occurs as alfalfa matures within cuttings over the whole growing season. Our objective was to determine the rate of change in alfalfa forage quality and yield during each of the periods in which it is harvested. In May, June, July, and September, 'Standfast', 'WL 346', and 'Affinity' alfalfa were harvested initially at late vegetative stage (stem length > 12 inches; no buds, flowers, or seed pods) in Wisconsin and Pennsylvania. Different plots of each variety were harvested every 5 days thereafter to 20 days of maturity. Forage yield and quality were measured for each harvest. Plots not harvested for yield and quality analysis for a particular period were cut at 1/10 bloom. Dry matter yield increased 100 lb/acre/day for the first cutting (May), 80 lb/acre/day for the second and third cutting (June and July) and 20 lb/acre/day for the fourth cutting (September). Forage quality of early cuttings changed faster than later cuttings; crude protein (CP) declined 0.3%/day in May compared to 0.2 %/day in June, July, and September, while neutral detergent fiber (NDF) increased 0.4%/day in May and June, 0.3%/day in July, and 0.2%/day in September. In contrast, neutral detergent fiber digestibility (NDFD) declined more rapidly during summer cuttings (0.45%/day in June and 0.35%/day in July) than during spring (0.15%/day) or fall (0.30%/day) cuttings. Given that variable environmental conditions greatly impact alfalfa yield and quality, our results suggest that tradeoffs between yield and quality are greatest early in the growing season when alfalfa is growing more quickly, while quality of harvests made later in the growing season are impacted less by maturity.